Bad news you need a new transmission. these were the first year type with o/d clutch pack instead of torque converter system. Simply put this is an internal problem complete rebuild.
I went through 2 of these both Ford OEM replacements the final solution was a Jasper Rebuild problem solved.
The Spirit of Thunderbird - 2005 was released on: USA: 2005 (Thuderbird Film Festival)
seepage will take place since pressure is not being released
Dinosapien - 2007 The Thunderbird 1-15 was released on: USA: 2007
Mystery Hunters - 2002 Amazons Thunderbird was released on: USA: 2 October 2004
Socially Disconnected Issues - 2012 was released on: USA: 1 June 2012
Lost Tapes - 2008 Thunderbird 1-10 was released on: USA: 3 February 2009
The Brothers Brannagan - 1960 Thunderbird 1-23 was released on: USA: 18 March 1961
Disconnected Images - 2006 was released on: USA: 8 October 2006 (Coney Island Film Festival)
Disconnected - 2010 Deb 1-1 was released on: USA: 1 October 2010
Disconnected Politics the Press and the Public - 2000 TV was released on: USA: 2 June 2000
Obscurity - 2012 Disconnected Part 1 1-3 was released on: USA: 2012
Obscurity - 2012 Disconnected Part 2 1-4 was released on: USA: 2012